1. Field of the Invention
The invention relates to a method and device for prospection in wells and particularly deviated wells, comprising the use of transmission-reception means located in the same well portion. The term deviated wells should be understood to mean wells that are at least partially horizontal or very highly inclined with respect to the vertical.
More particularly, the invention relates to the seismic recording of a subsoil zone through which a horizontal well portion passes, the investigation range and resolution being intermediate between those generally obtained with sonic well-logging probes and those which are possible using conventional seismic prospection methods at a great distance.
Well-logging tools generally comprise, in the same body, one or more transmission members capable of generating acoustic signals in a frequency range of about 20 to 25 kHz and one or more corresponding receiving members. Such tools are well adapted to the fine study of wells or their immediate environment. To obtain a great range, very low frequency seismic waves are usually used in the range between 10 and 200 Hz for example, with an attendant disadvantage of very poor resolution. The seismic pulse source is generally disposed on the surface and the waves reflected by the underground discontinuities are picked up by one or more geophones housed in the body of the probe lowered into a well.
It is interesting for geophysicists to have seismic recordings concerning a relatively extensive zone of the ground through which wells pass and particularly deviated wells which are drilled through sedimentary basins for better studying the configuration thereof. The intermediate range required makes it necessary to use acoustic waves of a frequency much lower than those of well-logging probes. But the most interesting results can only be obtained by significantly increasing the distance separating the acoustic wave transmitters and receivers. This may be achieved without too much difficulty in vertical wells or well portions using if required a well tool comprising at least one satellite probe suspended at a certain distance under a main probe. But such an arrangement becomes inoperative in the case of highly deviated wells and particularly in horizontal well portions because of the difficulties of advancing and handling the probes.
2. Description of the Prior Art
Intermediate frequency acoustic waves have already been used in wells. From U.S. Pat. No. 4,542,487, there is known a seismic prospection system comprising a source disposed at the surface which transmits acoustic waves in an intermediate frequency range up to 300 Hz. The waves are received by different geophones in the body of a well probe. With such an arrangement in which the source remains at the surface the useful depth of penetration of the probe is restricted for the highest frequencies of the frequency range considered. In addition, the device described does not lend itself to use in a very deviated well portion. U.S. Pat. No. 4,394,754 may also be mentioned as describing the use in a well of an assembly formed of a transmitter emitting acoustic vibrations at a frequency of 1 kHz and two receivers spaced apart by a few meters. It should be noted that such an assembly is not adapted either for operating in very deviated well portions and that the distance between the transmitter and the receivers is here very small.
In the European patent application 246 148 a seismic prospection method is described particularly applicable in well portions slightly inclined with respect to the horizontal. It comprises the use of one or more acoustic wave transmitters whose frequency is in the 5000 to 10000 Hz band and so below that in which well-logging probes generally operate. The waves reflected by the discontinuities surrounding the well are received by one or more piezoelectric sensors. The transmitters and receivers are disposed in a single well probe fixed at the end of a drill string and pushed as far as the deviated zone to be studied. The maximum spacing between the transmitters and receivers is about 10 meters or so. Such an arrangement is suitable for obtaining well logs in a widened zone about a well. But it lends itself poorly to seismic prospection operations at frequencies appreciably lower because the type of receivers used does not discriminate between the upgoing and downgoing waves. The relatively reduced spacing between the transmitters and receivers, imposed by their arrangement in a single probe, also provides a limitation.